1. Field of the Invention
The present invention relates to a headbox apparatus for a papermaking machine. More specifically, the present invention relates to a headbox apparatus that defines a flow path for stock flowing between an upstream header and a downstream slice lip.
2. Background Information
In a headbox of a papermaking machine, a vertical channel flow restriction device is located inside the headbox and distributes a uniform flow of fluid from the headbox delivery system to the headbox nozzle. The vertical channel flow restriction device is located between the pond sides and is trapped by the apron support structure and the top of the headbox nozzle. The fluid or stock is accelerated through the vertical channel openings into rectangular chambers located adjacent to one another. The discharge side of the vertical channel flow restriction device is nearly 100% open area into the headbox nozzle. The vertical openings provide a more uniform flow distribution requiring less mixing of individual flow streams and a uniform pressure drop across the distributor which produces a better basis weight profile.
The vertical channel flow restriction device is constructed from either metallic or non metallic material as the segmented design makes cross machine thermal expansion less critical than previous designs. The flow passages in the channel flow restriction device are smooth in order to prevent fiber from adhering to the surfaces.
In the headbox of the present invention, the channel flow restriction device can be constructed from multiple segments or from a single piece of material. When constructed from multiple pieces, the vertical channel flow restriction device can be welded or glued, or simply by using locating devices and clamping forces. The vertical channel flow restriction device can utilize a continuous channel from the apron floor to the slice roof, or can be divided into individual stratified channels to divide the flow for purposes of different stock supply, or the use of turbulence control vanes in the slice region of the headbox.
The multiple pieces stacked together may still be trough bolted for structural rigidity by placing each layer of flow restriction device above one another keeping the channels and solid areas in line.
The channel flow restriction device tending and drive side elements, or multiple tending and drive side elements near the tending and drive side walls, may include a mechanism that alters their position thus altering the flow in their adjacent channels. This flow alteration provides a tool for controlling the fluid velocity and volume thus altering the fiber orientation. This alteration may either be the entrance size of these end channels, or modification of the inlet condition of these end channels.
The restriction device segments are fixed in location inside the headbox by locating devices. The upstream surfaces have a series of vertical channels located on an equal pitch across the entire width of the headbox. The flow elements can be designed such that the width of the channel can be easily changed. Changing the channel width changes the velocity and pressure drop for a given flow entering the vertical channel flow restriction device resulting in improved flow distribution.
The vertical channel flow restriction device is designed such that the range of fluid velocities in the initial section of the tube bank is between 3 and 50 feet per second. The exiting velocity range from the vertical tube bank is 1 to 20 feet per second.
The vertical channel flow restriction device can be fed from either a cross machine header or multiple flow injection hoses and can be used in combination with a dilution control of the flow leading to the flow restriction segments, or with dilution control fed within the flow restriction segments themselves. When used with a cross machine header, the segments themselves may contain a series of holes or slots that can deliver dilution control water into the cross machine header into the flow channel, or into the expansion portion of the element.
More specifically, the headbox apparatus according to the present invention includes the following features:
1. The vertical flow restriction device minimizes the mixing requirements of multiple individual tubes.
2. The mixing of flows is primarily in the cross machine direction, reducing rotational flow vortexes and maximizes cross machine mixing.
3. The vertical flow restriction device is constructed from one or multiple pieces.
4. The vertical channel flow restriction device is constructed with a series of channels on equal or near equal pitch across the entire width of the headbox
5. The tending side and drive side elements include a mechanism to alter the flow rate through these channels either by width or entrance configuration.
6. The vertical channel width can be easily modified to increase or decrease the pressure drop across the flow restriction device.
7. The vertical tube bank is located inside the headbox in the wet end side of the nozzle between the pond sides, apron support structure and nozzle roof.
8. The vertical flow restriction device is constructed of multiple MD and CD direction adjacent zones with varying open areas.
9. The vertical flow restriction device discharge side has greater than 85% open area into the nozzle area of the headbox.
10. The vertical flow restriction device can be utilized in headboxes where the delivery of fluid to the headbox is completed by means of a tapered header or a multiple tube/hose delivery system.
11. The vertical flow restriction device can be operated in conjunction with a dilution control system or without a dilution control system
12. The vertical flow restriction device may contain multiple holes or channels to deliver dilution water through the element and into the tapered header or into other areas of the element.
More particularly, a conventional headbox distributor uses a tube array to spread the pulp slurry as uniformly as possible across the width of a paper machine headbox prior to the start of the drainage or other thickening process. The tube array is generally made up of individual round inlet tubes mounted is some manner to cause acceleration of the flow into each tube from a cross machine header or other form of supply of the slurry prior to the tube array. The pressure drop from the acceleration of the flow at the inlet of each tube within the array is critical to the uniformity of the flow within each tube and therefore to the uniformity of the cross machine uniformity of the headbox in general. This acceleration of the flow is also a factor in the operational cleanliness of the headbox operation as well as flow stability and uniformity. The exit end of a typical tube array may take on many shapes (round. hexagonal. rectangular or square or other) but eventually the flows exiting each individual tube must be re-joined prior to or within the nozzle of the headbox prior to discharge to the drainage area. The reorientation of the round tube entrance flow to the eventual rectangular shape of the nozzle will create disturbances in the flow in all directions. These disturbances must be damped or reduced in some way prior to discharge out the slice so as not to cause nonuniformities in the paper web.
The purpose of the present invention is to create the necessary pressure drop and subsequent uniform cross machine flow distribution using only vertical channels in the flow distributor. This will minimize non-cross machine flow disturbances improving cross machine uniformity of the flow. The use of only vertical channels also improves the cross machine characteristics of the fiber slurry by minimizing non cross machine forces on the fibers by the fluid flow. The design is equally applicable to headboxes using dilution to control the headbox profile or other mechanical profile control techniques.
Further advantages of this invention are simplicity of manufacturing using easily manufactured turbulence generating devices and improved structural stiffness within the headbox to withstand internal pressures of the fluid flow.
Therefore, the primary feature of the present invention is the provision of a headbox apparatus that overcomes the problems associated with the prior art headboxes and which makes a significant contribution to the papermaking art.
Another feature of the present invention is the provision of a headbox apparatus for a papermaking machine that is easier and less costly to manufacture when compared with prior art headboxes.
Other features and advantages of the present invention will be readily apparent to those skilled in the art by a consideration of the detailed description of a preferred embodiment of the present invention contained herein.